My old Parabees - the White Knights

[xcortes]

This weekend I took out whatever was left of the Parabees. Mostly gone except for the power supplies. Decided on how to rebuild using only tubes, iron and main parts on hand. The amps will be 2w 300Bs with dowdy chokes and tl404s. Alps pot in the front. All white to be used in my office (picked a pair of the cheap BS 22 Pioneer soeakers which are so far very promising). Driver stage is a c4sd 5965. Direct coupled. DC filaments using PJs old recipee. The paint job leaves a lot to be desired as I only left the painting curate a couple of hours. I had forgotten how nice the pgp 8.1s are.

[johnsonad]

What is your operating point?

[xcortes]

Same as a 45

[Paul Birkeland]

Driver stage is a c4sd 5965. Direct coupled.

Are you pulling plate voltage from the cathode of the 300B?  I don't see any power handling numbers for the TL-404, but I'm quite sure that aiming a little higher isn't going to hurt them.  With the high DCR of the Dowdy and the meager voltage already available, I would consider abandoning the direct coupling.

If you want to pull the driver plate voltage from the 300B cathode like the old amps used to, I'd throw my vote in for a choke loaded driver tube with adjustable bias.

[xcortes]

I could go cap coupled and choke load the driver. I have some exo-01 chokes that would work perfectly for that appltcation (the trick will be where to put them). Adjustable bias as in a pot doing the job of the cathode resistor? In the driver?

Pulling plate voltage from the cathode of the 300B meaning literally using the 300B cathode voltage to supply the driver?

[Paul Birkeland]

I could go cap coupled and choke load the driver. I have some exo-01 chokes that would work perfectly for that appltcation (the trick will be where to put them). Adjustable bias as in a pot doing the job of the cathode resistor? In the driver?

Yeah, that's one way to do it.  I don't know what bias voltage you're shooting for, but I would guess it is around 50V?  If so, the driver tube would have to suck down more current than the EXO-001 can handle to run straight off the cathode.  You can either take your driver voltage off the cathode resistor of the 300B, drop it down a bit, then use the EXO-001 (with a 6C45 maybe), or you can send some money to EML and use something like the 20A with the EXO-001 on just about the full B+ that the PGP8.1 has available (check with PJ or Mikey about the current available on the 6.3V winding).

[xcortes]

Yes, 50v bias. So the 6c45 would be operating with a 45 volts p-k voltage (the choke dcr is 1k)? And about 4mA at a 0.5 volts bias? Does that sound right? How about a 437A at the same conditions?

[Paul Birkeland]

The EXO-001 will take 20mA, and I'd suggest using as much of that as-is practical.  That gives you 20V of your 50V drop at most, and a good way to get the rest of the drop is to split up the cathode bias resistor on the output stage into two resistors, which can give you your extra 30V of drop.

I would suggest 1V of bias and 115V P-K, I suppose the question is, what is the target operating point for the output stage?  If there isn't sufficient voltage at the cathode of the output tube, there are some other directions you can go.

Here is a link to an amplifier using a similar idea:
http://www.audioasylum.com/forums/set/messages/6/66827.html

[xcortes]

I get it now. But this is a direct coupled amp isn't it?

[RPMac]

I've been looking at this schematic to modify my Paraglows.
I have a pair of BH-7s and wonder if they would work for the driver. I would like to have a DHT driver like 10y, 801a, 45 or 26. PB keeps pushing toward the obvious choice, EML 20a.

[Paul Birkeland]

I get it now. But this is a direct coupled amp isn't it?

Yeah.  Personally, I would go cap coupled and use all that lovely voltage that's available.  The 350V P-K/50mA/76V bias into a 5K load gives about 6 clean watts.

 I'm seeing about 471V of B+ available after CLC filtering via the RGC-6 choke (180R/10H/75mA).  471V B+ to start, take away 45V of drop through the dowdy, then take away 76V of bias, and you have 350V left for the 300B.  This seems a little too easy...

[xcortes]

The dowdy is specd for 40 mA and knowing Mike I'm sure they could do the 50. The 404s, otoh, are only specd for two watts.

What happens if you run a parafeed opt above its max power? Distortion or can it be damaged?

[johnsonad]

Thank you for that my friend! I may pick up a second pair of 404's and try them in my 5 watt 300B amps.

[Paul Joppa]

The power limitation (due to magnetic saturation) of an output transformer goes as frequency squared. For example, the Kaiju output transformer will handle 8 watts at 22Hz, or 32 watts at 44Hz.

(Note that this is unrelated to the small-signal frequency response.)

[xcortes]

Thanks Paul,

I chatted with Mike who confirmed that his published mols are conservative, that the effects from over driving a transformer are distortion and heat but nothing permanent so basucally you can use a 2 watter in a 6w circuit and it should be ok if you run it without passing the 2w continuouosly. So I'm gonna try!